as it was later discovered that a mistake had been made as to the missing man’s identity.' Now let's look at the rest of that sentence:

Note that the verbs are now active: 'needn't have caused' and 'telling.' That is, they no longer consist of a form of 'to be' plus a past participle ('caused' is a past participle, but it's preceded by auxiliary 'have,' not 'been' - 'have caused' is present perfect active voice). ' The army authorities needn't have caused him so much distress by telling him that his brother had died in action. The subject of the passive verbs 'needn't have been caused' and 'being told' is 'he,' and the agent is 'the army authorities.' So 'he' will become the object of the active verbs (in the object form 'him'), and 'the army authorities' will become the subject:

' He needn’t have been caused so much distress by being told by the army authorities that his brother had died in action. Let's look at the first part of your first sentence: If there is no agent, you have to invent a subject for the active verb. When you turn a passive sentence into an active one, the subject of the passive verb generally becomes the object of the active verb, and the agent (if there is one) becomes the subject of the active verb. In the passive voice, the subject of the verb has the action done to it, and the person/thing actually doing the action may be expressed in a prepositional phrase starting with 'by.' The person/thing doing the action is called the 'agent.' Note that the agent does not always appear in a passive voice sentence (this info may not be relevant).

Passive to active online converter